“Believe it or not, you and I are both in this photograph,” Prince William says of a photo of himself with a pregnant Princess Diana. “You’re in the tummy.”

In addition to sharing family photos, the brothers open up about how the late Princess of Wales was as a mother and how she balanced fun and the realities of life beyond the palace.

“This is the first time the two of us have ever spoken about her as a mother. She was very informal and really enjoyed the laughter and the fun. She was one of the naughtiest parents — but she understood that there was a real life outside of palace walls,” Prince William says.

Prince Harry echoes his brother’s fond memories of their mother, calling her “the best mum in the world.”

“She was our mum. She still is our mum. And of course, as a son I would say this, she was the best mum in the world. She smothered us with love, that’s for sure.”

The documentary comes in advance of the 20th anniversary of Princess Diana’s death in a Paris car crash in 1997. It will air on HBO later this month.